Cold cathode fluorescent lamp (CCFL) has many advantages such as higher efficiency, longer life span, lower cost and the like, thus is widely used on backlight modules of display devices, game devices, office equipments and lighting fixtures. To enable the CCFL to emit different colors of light different types of gases or fluorescent materials have to be filled in the CCFL. As a result the duty voltage of different colors of CCFL also varies. The CCFL emitting blue light (called “blue lamp” in short hereinafter) has a higher load impedance. If the voltage to actuate the blue lamp is not adequate, the lamp has a section of “dark segment” which is darker. This causes a lower luminosity or color variation in the backlight modules or devices it is adopted. To enable the blue lamp to be actuated normally, circuits adopted piezoelectric transformers to drive the CCFL have been developed in prior art. For instance R.O.C. patent No. I266277 entitled “Current control apparatus for fluorescent lamps” granted on Nov. 11, 2006 is one of them. As the impedance at the start time of the blue lamp is greater, and the piezoelectric transformer has characteristics mating the lamp impedance, the piezoelectric transformer outputs a higher instant voltage to mate the impedance. The impedance of the blue lamp decreases slightly after the blue lamp has been actuated, then the output voltage of the piezoelectric transformer drops to a regular duty voltage. FIG. 1 shows an output waveform chart of a circuit adopted in a conventional piezoelectric transformer. It indicates that the instant output voltage of the piezoelectric transformer at the start time increases rapidly. The instant voltage could be too high and trigger a protection mechanism of an inverter that supplies electric power, and cause a forced shutdown of the inverter. Moreover, impact of the excessive high voltage shortens the life span of electrodes in the blue lamp. Hence although the piezoelectric transformer can provide a sufficient voltage for the blue lamp corresponding to the impedance, it also has drawbacks such as producing an excessive high instant voltage at the start time to trigger the protection mechanism or causing damage of the electrodes in the lamp.